Predicting the catalytic efficiency by quantum-chemical descriptors: Theoretical study of pincer metallic complexes involved in the catalytic Heck reaction

A tool to predict the catalytic activity by interpolation was constructed, correlating a quantum chemical descriptor like absolute hardness with the catalytic activity experimentally measured in turnover numbers (TONs) for pincer metallic complexes. The linear relationship showed its usefulness reproducing correctly the magnitude order of TON for a catalyst reported in the literature. From the two quantum chemical descriptors considered in the present study, atomic charge on M and absolute hardness, the best correlation observed with the experimental catalytic activity corresponded to the case when the absolute hardness (calculated as (εLUMO − εHOMO)/2) was involved, being this an evidence of the orbital-control present in these kind of complexes. After systematic modifications to the general structure of a pincer complex, it was observed that the presence of P as heteroatom and Cl as leaving group gives a good compromise in terms of absolute hardness and charge on the metal center. The symmetry of the frontier orbitals is also an important issue to take into account: HOMOs with dz2dz2-like orbital symmetry and LUMOs with dxy-like orbital symmetry favor nucleophilic attacks and octahedral entrance of ligands.